Usages of this class should typically be of the form: * {@code ThreadLocalRandom.current().nextX(...)} (where * {@code X} is {@code Int}, {@code Long}, etc). * When all usages are of this form, it is never possible to * accidentally share a {@code ThreadLocalRandom} across multiple threads. * *
This class also provides additional commonly used bounded random * generation methods. * *
Instances of {@code ThreadLocalRandom} are not cryptographically
* secure. Consider instead using {@link java.security.SecureRandom}
* in security-sensitive applications. Additionally,
* default-constructed instances do not use a cryptographically random
* seed unless the {@linkplain System#getProperty system property}
* {@code java.util.secureRandomSeed} is set to {@code true}.
*
* @since 1.7
* @author Doug Lea
*/
@RandomGeneratorProperties(
name = "ThreadLocalRandom",
i = 64, j = 0, k = 0,
equidistribution = 1
)
public final class ThreadLocalRandom extends Random {
/*
* This class implements the java.util.Random API (and subclasses
* Random) using a single static instance that accesses 64 bits of
* random number state held in class java.lang.Thread (field
* threadLocalRandomSeed). In doing so, it also provides a home
* for managing package-private utilities that rely on exactly the
* same state as needed to maintain the ThreadLocalRandom
* instances. We leverage the need for an initialization flag
* field to also use it as a "probe" -- a self-adjusting thread
* hash used for contention avoidance, as well as a secondary
* simpler (xorShift) random seed that is conservatively used to
* avoid otherwise surprising users by hijacking the
* ThreadLocalRandom sequence. The dual use is a marriage of
* convenience, but is a simple and efficient way of reducing
* application-level overhead and footprint of most concurrent
* programs. Even more opportunistically, we also define here
* other package-private utilities that access Thread class
* fields.
*
* Even though this class subclasses java.util.Random, it uses the
* same basic algorithm as java.util.SplittableRandom. (See its
* internal documentation for explanations, which are not repeated
* here.) Note that ThreadLocalRandom is not a "splittable" generator
* (it does not support the split method), but it behaves as if
* one instance of the SplittableRandom algorithm had been
* created for each thread, each with a distinct gamma parameter
* (calculated from the thread id).
*
* Because this class is in a different package than class Thread,
* field access methods use Unsafe to bypass access control rules.
* To conform to the requirements of the Random superclass
* constructor, the common static ThreadLocalRandom maintains an
* "initialized" field for the sake of rejecting user calls to
* setSeed while still allowing a call from constructor. Note
* that serialization is completely unnecessary because there is
* only a static singleton. But we generate a serial form
* containing "rnd" and "initialized" fields to ensure
* compatibility across versions.
*
* Implementations of non-core methods are mostly the same as in
* SplittableRandom, that were in part derived from a previous
* version of this class.
*
* This implementation of ThreadLocalRandom overrides the
* definition of the nextGaussian() method in the class Random,
* and instead uses the ziggurat-based algorithm that is the
* default for the RandomGenerator interface.
*/
private static int mix32(long z) {
z = (z ^ (z >>> 33)) * 0xff51afd7ed558ccdL;
return (int)(((z ^ (z >>> 33)) * 0xc4ceb9fe1a85ec53L) >>> 32);
}
/**
* Field used only during singleton initialization.
* True when constructor completes.
*/
boolean initialized;
/** Constructor used only for static singleton */
private ThreadLocalRandom() {
initialized = true; // false during super() call
}
/**
* Initialize Thread fields for the current thread. Called only
* when Thread.threadLocalRandomProbe is zero, indicating that a
* thread local seed value needs to be generated. Note that even
* though the initialization is purely thread-local, we need to
* rely on (static) atomic generators to initialize the values.
*/
static final void localInit() {
int p = probeGenerator.addAndGet(PROBE_INCREMENT);
int probe = (p == 0) ? 1 : p; // skip 0
long seed = RandomSupport.mixMurmur64(seeder.getAndAdd(SEEDER_INCREMENT));
Thread t = Thread.currentThread();
U.putLong(t, SEED, seed);
U.putInt(t, PROBE, probe);
}
/**
* Returns the current thread's {@code ThreadLocalRandom} object.
* Methods of this object should be called only by the current thread,
* not by other threads.
*
* @return the current thread's {@code ThreadLocalRandom}
*/
public static ThreadLocalRandom current() {
if (U.getInt(Thread.currentThread(), PROBE) == 0)
localInit();
return instance;
}
/**
* Throws {@code UnsupportedOperationException}. Setting seeds in
* this generator is not supported.
*
* @throws UnsupportedOperationException always
*/
public void setSeed(long seed) {
// only allow call from super() constructor
if (initialized)
throw new UnsupportedOperationException();
}
/**
* Update the thread local seed value by adding to it the sum
* of {@code GOLDEN_GAMMA} (an odd value) and twice the thread id.
* This sum is always odd (to guarantee that the generator
* has maximum period) and is different for different threads.
* Because thread id values are allocated consecutively starting
* from 0, the high 32 bits of this sum will be the same as the
* high 32 bits of {@code GOLDEN_GAMMA} unless an extremely large
* number of threads have been created, and so the overall
* value added to the thread local seed value will have at least
* fourteen 01 and 10 transitions (see the documentation for the
* method {@code mixGamma} in class {@code SplittableRandom}),
* which should provide adequate statistical quality for
* applications likely to use {@code ThreadLocalRandom}.
*/
final long nextSeed() {
Thread t; long r; // read and update per-thread seed
U.putLong(t = Thread.currentThread(), SEED,
r = U.getLong(t, SEED) + (t.getId() << 1) + GOLDEN_GAMMA);
return r;
}
/**
* Generates a pseudorandom number with the indicated number of
* low-order bits. Because this class has no subclasses, this
* method cannot be invoked or overridden.
*
* @param bits random bits
* @return the next pseudorandom value from this random number
* generator's sequence
*/
protected int next(int bits) {
return nextInt() >>> (32 - bits);
}
// Within-package utilities
/*
* Descriptions of the usages of the methods below can be found in
* the classes that use them. Briefly, a thread's "probe" value is
* a non-zero hash code that (probably) does not collide with
* other existing threads with respect to any power of two
* collision space. When it does collide, it is pseudo-randomly
* adjusted (using a Marsaglia XorShift). The nextSecondarySeed
* method is used in the same contexts as ThreadLocalRandom, but
* only for transient usages such as random adaptive spin/block
* sequences for which a cheap RNG suffices and for which it could
* in principle disrupt user-visible statistical properties of the
* main ThreadLocalRandom if we were to use it.
*
* Note: Because of package-protection issues, versions of some
* these methods also appear in some subpackage classes.
*/
/**
* Returns the probe value for the current thread without forcing
* initialization. Note that invoking ThreadLocalRandom.current()
* can be used to force initialization on zero return.
*/
static final int getProbe() {
return U.getInt(Thread.currentThread(), PROBE);
}
/**
* Pseudo-randomly advances and records the given probe value for the
* given thread.
*/
static final int advanceProbe(int probe) {
probe ^= probe << 13; // xorshift
probe ^= probe >>> 17;
probe ^= probe << 5;
U.putInt(Thread.currentThread(), PROBE, probe);
return probe;
}
/**
* Returns the pseudo-randomly initialized or updated secondary seed.
*/
static final int nextSecondarySeed() {
int r;
Thread t = Thread.currentThread();
if ((r = U.getInt(t, SECONDARY)) != 0) {
r ^= r << 13; // xorshift
r ^= r >>> 17;
r ^= r << 5;
}
else if ((r = mix32(seeder.getAndAdd(SEEDER_INCREMENT))) == 0)
r = 1; // avoid zero
U.putInt(t, SECONDARY, r);
return r;
}
// Support for other package-private ThreadLocal access
/**
* Erases ThreadLocals by nulling out Thread maps.
*/
static final void eraseThreadLocals(Thread thread) {
U.putReference(thread, THREADLOCALS, null);
U.putReference(thread, INHERITABLETHREADLOCALS, null);
}
static final void setInheritedAccessControlContext(Thread thread,
@SuppressWarnings("removal") AccessControlContext acc) {
U.putReferenceRelease(thread, INHERITEDACCESSCONTROLCONTEXT, acc);
}
// Serialization support
private static final long serialVersionUID = -5851777807851030925L;
/**
* @serialField rnd long
* seed for random computations
* @serialField initialized boolean
* always true
*/
private static final ObjectStreamField[] serialPersistentFields = {
new ObjectStreamField("rnd", long.class),
new ObjectStreamField("initialized", boolean.class),
};
/**
* Saves the {@code ThreadLocalRandom} to a stream (that is, serializes it).
* @param s the stream
* @throws java.io.IOException if an I/O error occurs
*/
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
java.io.ObjectOutputStream.PutField fields = s.putFields();
fields.put("rnd", U.getLong(Thread.currentThread(), SEED));
fields.put("initialized", true);
s.writeFields();
}
/**
* Returns the {@link #current() current} thread's {@code ThreadLocalRandom}.
* @return the {@link #current() current} thread's {@code ThreadLocalRandom}
*/
private Object readResolve() {
return current();
}
// Static initialization
/**
* The seed increment. This must be an odd value for the generator to
* have the maximum period (2 to the 64th power).
*
* The value 0x9e3779b97f4a7c15L is odd, and moreover consists of the
* first 64 bits of the fractional part of the golden ratio,
* which is known to generate good Weyl sequences.
*/
private static final long GOLDEN_GAMMA = 0x9e3779b97f4a7c15L;
/**
* The increment for generating probe values.
*/
private static final int PROBE_INCREMENT = 0x9e3779b9;
/**
* The increment of seeder per new instance.
*/
private static final long SEEDER_INCREMENT = 0xbb67ae8584caa73bL;
// IllegalArgumentException messages
static final String BAD_BOUND = "bound must be positive";
static final String BAD_RANGE = "bound must be greater than origin";
static final String BAD_SIZE = "size must be non-negative";
// Unsafe mechanics
private static final Unsafe U = Unsafe.getUnsafe();
private static final long SEED
= U.objectFieldOffset(Thread.class, "threadLocalRandomSeed");
private static final long PROBE
= U.objectFieldOffset(Thread.class, "threadLocalRandomProbe");
private static final long SECONDARY
= U.objectFieldOffset(Thread.class, "threadLocalRandomSecondarySeed");
private static final long THREADLOCALS
= U.objectFieldOffset(Thread.class, "threadLocals");
private static final long INHERITABLETHREADLOCALS
= U.objectFieldOffset(Thread.class, "inheritableThreadLocals");
private static final long INHERITEDACCESSCONTROLCONTEXT
= U.objectFieldOffset(Thread.class, "inheritedAccessControlContext");
/** Generates per-thread initialization/probe field */
private static final AtomicInteger probeGenerator = new AtomicInteger();
/** The common ThreadLocalRandom */
private static final ThreadLocalRandom instance = new ThreadLocalRandom();
/**
* The next seed for default constructors.
*/
private static final AtomicLong seeder
= new AtomicLong(RandomSupport.mixMurmur64(System.currentTimeMillis()) ^
RandomSupport.mixMurmur64(System.nanoTime()));
// at end of